JavaScript function to test (a bit brute force test) how much space is allocated for localStorage in a browser, if there are already entries in the localStorage it will show how much space is left.

testlocalstorage.js

    /**
    * Function to test how much space is allocated for localStorage in a browser, 
    * if there are already entries in the localStorage it will show how much space is left. 
    * console.logs commented out because IE is tripping over them. 
    */
    function testLocalStorage(){
        var timeStart = Date.now();
        var timeEnd, countKey, countValue, amountLeft, itemLength;
        var occupied = leftCount = 0; 
    //create localStorage entries until localStorage is totally filled and browser issues a warning.
    var i=0;
    while (!error){
     try {
        //length of the 'value' was picked to be a compromise between speed and accuracy, 
        // the longer the 'value' the quicker script and result less accurate. This one is around 2Kb 
        localStorage.setItem('testKey'+i, '11111111112222222222333333333344444444445555555555666661111111111222222222233333333334444444444555555555566666');
    } catch (e) {
       var error = e;
         //console.log('error: '+ e);
     }
     i++;
 }
 //if the warning was issued - localStorage is full.
 if (error){
        //iterate through all keys and values to count their length
        for (var i = 0; i < localStorage.length; i++){
            countKey = localStorage.key(i);
            countValue = localStorage.getItem(localStorage.key(i));
            itemLength = countKey.length + countValue.length;
            //if the key is one of our 'test' keys count it separately
            if(countKey.indexOf("testKey") !== -1){
                leftCount = leftCount + itemLength;
            }
            //count all keys and their values
            occupied = occupied + itemLength;
        };
        //all keys + values lenght recalculated to Mb
        occupied = (((occupied*16)/(8*1024))/1024).toFixed(2); 
        //console.log("total localStorage: "+occupied+"Mb");
        //if there are any other keys then our 'testKeys' it will show how much localStorage is left
        amountLeft = occupied - (((leftCount*16)/(8*1024))/1024).toFixed(2);
        //console.log("localStorage left: "+amountLeft+"Mb"); 
        //iterate through all localStorage keys and remove 'testKeys'
        Object.keys(localStorage).forEach(function(key){
         if (key.indexOf("testKey") !== -1) {
             localStorage.removeItem(key);
         }
     });
        
    }
    //calculate execution time
    var timeEnd = Date.now();
    var time = timeEnd - timeStart;
    //create message
    var message = 'Finished in: ' + time + 'ms \n total localStorage: '+occupied+'Mb \n localStorage left: '+amountLeft+"Mb";
    //put the message on the screen
    document.getElementById('scene').innerText = message; //this works with Chrome,Safari, Opera, IE
    //document.getElementById('scene').textContent = message;  //Required for Firefox to show messages
    //console.log(message);
}

tests_in_different_browsers.txt

Tests of testLocalStorage in different browsers on different platforms.

GalaxyTab 10.1 
Maxthon Pad 1.7 ~1130ms 5Mb
Firefox 20.0(Beta 20.0) crashed both
Chrome 25.0.1364.169 ~22250ms /5Mb
Native (identifies as Safari 4.0/Webkit534.30) ~995ms /5Mb

iPhone 4s iOS 6.1.3
Safari ~ 520ms /5Mb
As HomeApp ~525ms / 5Mb  
iCab ~ 710ms /5mb

MacBook Pro OSX 1.8.3 (Core 2 Duo 2.66 8Gb memory)
Safari 6.0.3 ~105ms /5Mb
Chrome 26.0.1410.43 ~3400ms /5Mb
Firefox 20.0 300150ms(!)  /10Mb (after complaining about script running to long)

iPad 3 iOS 6.1.3
Safari ~430ms /5Mb
iCab ~595ms /5mb

Windows 7 -64b (Core 2 Duo 2.93 6Gb memory)
Safari 5.1.7 ~80ms /5Mb
Chrome 26.0.1410.43 ~1220ms /5Mb
Firefox 20.0  228500ms(!)  /10Mb (after complaining about script running to long)
IE9 ~17900ms /9.54Mb ( if any console.logs are in the code does not work until DevTools are opened)
Opera 12.15 ~4212ms /3.55Mb (this is when 5Mb is selected, but Opera asks nicely if we want increase the amount of lS, unfortunately it crashes if test conducted a few times in a row)

Win 8 (Under Parallels 8)
IE10 ~7850ms /9.54Mb